The surface structure of alkali doped Si(111): B ultra-thin films has been studied by low-energy electron diffraction (LEED), X-ray photoemission spectroscopy (XPS) and scanning tunneling microscopy (STM). A comparative study of K/Si(111)-3 x 1 and K/Si(111):B-2 root 3 x 2 root 3R30 interfaces allowed us to determine the saturation coverage to be 0.5 monolayer in the later case. The 2 root 3-surface reconstruction is shown to be a common property of pure K, Rb, Cs materials and K0.4Rb0.6 alloys but progressively disappears if Rb is replaced by Ca. Taking into account the existence of two distinct boron sites in the ratio 1/3 as seen from B-1s core levels spectra, LAPW-DFT calculations have been carried out in order to optimize the atomic structure. As a result, alkali adatoms are shown to form trimers leading to a large modulation of the Si B bonds accompanied by an inhomogeneous doping of the dangling bonds in agreement with voltage dependent STM images. (C) 2012 Elsevier B. V. All rights reserved.